In a nutshell
Virtual Serial Port Kit creates virtual COM ports and connects each pair of them via virtual null-modem cable.
So, all data written to one virtual serial port can be immediately read from the other one, and vice versa.
How it works?
Using Virtual Serial Port Kit you can emulate unlimited number of serial ports and connect each pair of them via virtual null-modem cable.
So, your applications can exchange data just like they do it via hardware serial ports and real null-modem cables.
For example, using VSPK, you can create pair of virtual serial ports (e.g. COM7 and COM8) and connect two applications to them. All data that the first application writes to COM7 is read from COM8 by the second application, and vice versa.
It is easy to test. You can run two copies of terminal program (e.g. standard Windows HyperTerminal) and connect them to COM7 and COM8.
So, all you type in the first HyperTerminal will appear in the second and vice versa. You can also transfer files between HyperTerminals and do whatever you want.
Full COM ports emulation
Virtual serial ports fully emulate behaviour of hardware COM ports, so any serial communications application works with virtual serial port as if it was a hardware COM port.